The present invention relates to deep bed sand filters, and more particularly, to an upflow type of deep bed sand filter.
For many years, deep beds of sand, or other fine particulates, have been utilized for filtering relatively large quantities of liquid, for example, the secondary treatment of industrial waste water or the like. In the most primitive deep bed sand filters, the filtered solids which have been "trapped" among the sand particulates, can be removed from the filter only by throwing away the "dirty" sand.
In more advanced filters, the filter media has been conserved by a variety of techniques most-of which draw the dirty sand from the bottom of the filter vessel and pass the dirty sand through a media washing chamber situated either externally or internally of the vessel.
In one type of regenerative filter, the influent is introduced in the upper portion of the filter vessel above the sand bed, and the flow of both liquid and sand is in a downward direction in the vessel. This is represented by the filters described in U.S. Pat. No. 4,060,484 issued Nov. 29, 1977, and U.S. Pat. No. 4,891,142 issued Jan. 2, 1990.
Another approach, represented by U.S. Pat. No. 4,126,546 issued Nov. 21, 1978, and U.S. Pat. No. 4,246,102 issued Jan. 20, 1981 introduces the influent at an intermediate elevation within the sand bed, such that the fluid flows upwardly in the vessel, while the sand moves downwardly.
The patents mentioned above, as well as others such as U.S. Pat. No. 5,019,278 issued May 28, 1991, disclose a variety of said washing techniques, some of which use a portion of the filtrate as the washing medium. It can readily be appreciated that the use of filtrate in this manner, reduces the amount of filtrate that can be drawn from the filter vessel as "clean" water for subsequent, higher value use. In other words, known regenerative deep bed sand filters of the type that use filtrate as the wash medium, have an excessive percentage of the filtrate "wasted" in the reject flow.